The present invention relates to a system for transmitting the power of an engine to four wheels of a four-wheel drive vehicle and more particularly to an arrangement of a central differential in the system.
In a conventional four-wheel drive vehicle having a central differential, which is called a full time four-wheel drive vehicle, the central differential operates to uniformly distribute the torque of an engine to front wheels and to rear wheels of the vehicle. In order to provide a compact transmission system and to decrease the height of the vehicle, the central differential is disposed inside a transmission case.
Japanese Patent Laid-Open No. 59-81224 discloses such a transmission system for a vehicle having a laterally mounted engine. In the system, a planetary gear device as a central differential is disposed in alignment with an output shaft of the transmission at a side opposed to the engine with respect to the transmission. A sun gear of the planetary gear device is connected to front wheels through a shaft co-axially provided in the output shaft. A carrier is connected to the rear wheels through reduction gears, offset shaft, and so forth.
However, in the system of the prior art, since the central differential is disposed at a side of the transmission opposite to the engine and away from the center line with respect to the width of the vehicle, long shafts are necessary for transmitting the power from the central differential to the front and rear transmission systems. Thus, the construction of the transmission system becomes complicated. Furthermore, in some vehicles, overdrive speed gears cannot be easily added to the transmission because the central differential takes up the space for them.